1. Field of the Invention
This invention relates to a speaker for transducing electrical signals into an acoustic sound, and a speaker apparatus employing this speaker.
2. Description of the Related Art
There has hitherto been proposed a speaker apparatus having a magnetic circuit in which a pair of magnets are arranged with the magnetic poles of repulsive polarities facing each other with a center plate in-between to form a repellant magnetic field. Typical of the speaker apparatus of this sort is shown in Japanese Laying-Open Patent H-6-233383. Referring to FIG. 1, a speaker apparatus 101 disclosed in this publication includes a vibrating system 105 including a vibrating plate, a magnetic circuit 106 for driving the vibrating system 105, and a frame 107 for supporting the vibrating system 105 and the magnetic circuit 106. Referring to FIG. 1, this vibrating system 105 includes a substantially frusto-conical main vibrating plate 110, having a center through-hole, a substantially frusto-conical sub-vibrating plate 111, mounted on the main vibrating plate 110 with its center axis coincident with the main vibrating plate 110, a dome-shaped cap 112, provided for closing the through-hole of the main vibrating plate 110, a cylindrically-shaped bobbin 113, connected to the center of the main vibrating plate 110, a resilient edge 114 connected to the outer edge of the main vibrating plate 110, and a flexible vibration-absorbing damper 115 provided on the bobbin 113.
Still referring to FIG. 1, the magnetic circuit 106 includes a voice coil 119 fed with the driving current proportionate to the acoustic signals driving the vibrating system 105, a center plate 120 constituting a magnetic path and a pair of magnets 121, 122 providing magnetic flux to the center plate 120. The voice coil 119 is placed around the outer rim of the bobbin 113 of the vibrating system 105 and is provided on the outer rim of the center plate 120 and the magnets 121, 122. The voice coil 119 has its both ends connected via braided wires to a connecting portion, not shown, provided on the outer rim of the frame 107. The center plate 120 is formed as a disc from a magnetic material. The magnets 121, 122 are formed as discs and are arranged with the centers thereof coincident with the center of the center plate 120, with the center plate 120 in-between, so that the magnetic poles of repulsive polarities face each other. Referring to FIG. 1, a holder 108 for holding the main vibrating plate 110 is provided at an opening end of the frame 107. To this holder 108 is secured the outer rim of the edge of the main vibrating plate 110 via a ring-shaped gasket, 114. At a mid portion on the bottom surface of the frame 107 is formed a supporting lug for supporting the magnetic circuit 106.
If the driving current is fed to the voice coil 119 of the above-described speaker apparatus 101, the voice coil 119 is vibrated in accordance with Flemings left hand rule. The main vibrating plate 110 and the sub-vibrating plate 111 are vibrated as a result of the vibrations of the voice coil 119.
There is also known a speaker apparatus as disclosed in Japanese Laying-Open Patent H-6-233384. This speaker apparatus 102 includes a vibrating system 125, having, a magnetic circuit 126 for driving the vibrating system 125, a magnetic circuit 126 for driving the vibrating system 125 and a pair of frames 127, 128 supporting the vibrating system 125 and the magnetic circuit 126, as shown in FIG. 2. The vibrating system 125 includes a substantially flat-plate-shaped vibrating plate 130, having a center through-hole, a cylindrically-shaped bobbin 131, placed within the through-hole of the vibrating plate 130, a dome-shaped cap 132, closing one opening end of the bobbin 131, a resilient supporting member 133, mounted in continuation to the outer rim of the vibrating plate 130, and a pair of resilient vibration-absorbing dampers 134, 135, attached to the bobbin 131, as shown in FIG. 2.
The magnetic circuit 126 includes a voice coil 139, fed with the driving current proportionate to acoustic signals used for the driving system 125, a center plate 140 constituting the magnetic path, and a pair of magnets 141, 142 for providing the magnetic flux to the center plate 140. The voice coil 139 is placed around the outer periphery of the bobbin 131 of the vibrating system 125, and is arranged on the outer periphery of the bobbin 131 of the vibrating system 125, as shown in FIG. 2. The voice coil 139 has its both ends connected via a braided wire to a connecting portion, not shown, provided on the outer periphery of the frame 127. The center plate 140 is formed in a disc shape from a magnetic material, The magnets 141, 142 are formed in a disc shape and are arranged with the center axes thereof coincident with the center of the center plate 140, on both sides of the center plate 140, so that the magnetic poles of repulsive polarities will face each other, with the center plate 140 in-between.
The frames 127, 128 are assembled with each other to support the vibrating plate 130 and the magnetic circuit 126 therein, as shown in FIG. 2. The frames 127, 128 are each formed with a holder 129 for holding the vibrating plate 130 with the outer rim portion of the vibrating plate 130 being supported by the holder 129 via a resilient supporting member 133. A supporting lug for supporting the magnetic circuit 126 is formed at a mid portion on the bottom surface of the frame 128.
The above-described speaker apparatus 102 has its voice coil 139 fed with the driving current proportionate to the acoustic signals, whereby the voice coil 139 is vibrated in accordance with Flemings left hand rule. The vibrating plate 130 is vibrated with vibrations of the voice coil 139 to generate the acoustic sound.
There is another type of the speaker apparatus disclosed in Japanese Laying-Open Patent H-6-284499. This speaker apparatus 103 includes a vibrating system 145, including, a magnetic circuit 146 for driving the vibrating system 145 and a pair of frames 147, 148 for supporting the vibrating system 145 and the magnetic circuit 146, as shown in FIG. 3.
The vibrating system 145 has a set of substantially planar plate-shaped vibrating plates 150, 151, each having a center through-hole, and a resilient supporting member 153 mounted on the outer periphery of the vibrating plates 150, 151, as shown in FIG. 3. The vibrating plates 150, 151 are bonded to each other with a pre-set gap in-between. The resilient supporting member 153, carrying at its one end the vibrating plates 150, 151, has its other end mounted on the frames 147, 148.
Referring to FIG. 3, the magnetic circuit 146 includes a voice coil 159, fed with the driving current proportionate to the acoustic signals driving the vibrating system 125, a center plate 160 constituting the magnetic path, and a pair of magnets 161, 162 providing the magnetic flux to this center plate 160. The voice coil 159 is provided providing the inner periphery of the vibrating plates 150,151 of the vibrating system 145, and is arranged on the outer rim portions of the center plate 160 and the magnets 161, 162. These magnets 161, 162 are formed in a ring shape and are arranged with the center axes thereof coincident with the center of the center plate 160, on both sides of the center plate 160, so that the magnetic poles of repulsive polarities will face each other, with the center plate 160 in-between.
The frames 147, 148, having the same shape, are assembled with each other to support the vibrating system 145 and the magnetic circuit 146 therein. The frames 147, 148 are each formed with a holder 149 for holding the vibrating plates 150, 151. To this holder 149 are secured outer rims of the vibrating plates 150, 151 via resilient supporting member 153. The frames 147, 148 are each formed at mid portions of the bottom surfaces thereof with supporting lugs, not shown, adapted for supporting the magnetic circuit 126.
The above-described speaker apparatus 103 has its voice coil 159 fed with the driving current proportionate 'to the acoustic signals, whereby the voice coil 159 is vibrated in accordance with Flemings left hand rule. The vibrating plates 150, 151 are vibrated with vibrations of the voice coil 159 to generate the acoustic sound.
The magnetic circuits 106, 126, 146, making up the speaker apparatus 101, 102, 103, respectively, are effective to reduce the size of the entire apparatus.
The speaker apparatus 101 of FIG. 1, reduced in thickness by the magnetic circuit 106, has the bobbin 113 supported by the damper 115, and the sub-vibrating plate 111 is provided on the bobbin 113, so that it has a drawback that the vibrating system 105 has an increased thickness in the direction along the amplitude of the vibrating system 105.
The speaker apparatus 102 of FIG. 2, reduced in thickness by the magnetic circuit 126, is of a structure in which the dampers 134, 135 and the vibrating plate 130 are stacked along the amplitude direction of the vibrating system 125, so that it has a drawback that it has an increased thickness along the amplitude direction.
The speaker apparatus 103 of FIG. 3, reduced in thickness by the magnetic circuit 146, is not provided with dampers adapted for holding the voce coil 159 at a pre-set position in the magnetic field, so that it is susceptible to rolling on vibration of the vibrating plate 150 with an increased amplitude, with the result that it cannot reproduce the acoustic sound to high sound quality. This speaker apparatus 103 has a drawback that the voice coil 159 tends to be destroyed due to rolling of the vibrating plate 150 such that the apparatus is poor in resistance against high input. The speaker apparatus also has a drawback that air in an enclosure tends to leak from the gap between the voice coil 159 and the center plate 160, thereby distorting the reproduced sound.
The magnetic flux distribution in the magnetic circuits 106, 126, 146 of the speaker apparatus 101 to 103 is explained with reference to FIG. 4, in which the ordinate denotes the position along the thickness direction of a magnetic circuit parallel to the amplitude direction of the vibrating system and the abscissa denotes the orientation of the magnetic flux. Also, in FIG. 4, the arrows indicate the direction of the magnetic field. In the magnetic circuits 106, 126, 146, provided in the speaker apparatus 101, 102, 103, respectively, the magnetic flux density is maximum in the vicinity of the outer rim portions of the center plates 120, 140, 160, respectively, as shown in FIG. 4. The magnetic circuits 106, 126, 146 are designed so that the center along the thickness direction of the center plates 120, 140, 160 represents the position P3 having the maximum magnetic flux density.
With the magnetic circuits 106, 126, 146, the magnetic flux is decreased gradually in the direction from outer rim portions of the center plates 120, 140, 160 to the outer rims towards the magnets 121, 122, 141, 142, 161, 162, away from the center plates, with the magnetic flux becoming zero at mid portions along the direction of thickness of the outer rims of the magnets 121, 122, 141, 142, 161, 162. Also, with the magnetic circuits 106, 126, 146, the direction of the magnetic field indicated by arrows in FIG. 4 is inverted in a direction proceeding from the mid portion along the thickness of the outer rims of the magnets 121, 122, 141, 142, 161, 162 away from the center plates 120, 140, 160, with the magnetic flux in the reverse direction becoming maximum at the ends of the magnets 121, 122, 141, 142, 161, 162 and becoming zero at a position further away from the center plates 120, 140, 160.
In a large amplitude speaker apparatus, shown in FIGS. 5A to 5C, the length along the amplitude direction needs to be set to a larger value so that the magnetic flux density on the voice coil 170 mounted on the vibrating plate 172 will be constant irrespective of the position of the vibrating voice coil 170 along the amplitude direction, as shown in FIGS. 5A to 5C. Thus, it is difficult to reduce the thickness of the apparatus. With the speaker apparatus, having the magnetic flux distribution shown in FIG. 4, the magnetic flux lines having the reverse direction in a region on both sides along the direction of thickness of the center plate 171 act for obstructing the movement of the voice coil 170 to render it difficult to produce optimum acoustic characteristics.